The evolutionary reason why fish don't swim upside down

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A group of African catfish have fully embraced upside-down life. Their behavior is the result of a bottom-dwelling fish adapting to take advantage of oxygen-rich water near the surface. A catfish’s mouth is on its underside, so it was likely easiest for the fish to simply turn upside down and treat the top of the water the same as it used to treat the bottom says Lauren Chapman, who studies fish respiration at McGill University in Montreal, Quebec. “To an inverted catfish, the water’s surface is just another substrate,” she says.